This invention pertains to a control system of controlling an electric motor used for a drive unit for an electric vehicle such as an electric scooter, an electric car or the likes.
In general, an electric motor comprises a rotor having a field system and a stator having n phases armature coils (n is two or more integers) and is adapted to control a rotational speed by controlling a drive current flowing through the armature coils by means of a controller. Of late, there has been used one having a microprocessor provided as the controller.
In many cases, a brushless DC motor has been used for the electric motor driving the electric vehicle. As well known, the brushless DC motor comprises a rotor having a magnet field system and a stator having armature coils of multi-phases such as two or more phases. The rotor is rotated by switching an exciting phase of the armature coils to selectively energize the armature coils in accordance with a rotational angle position of the rotor relative to the stator.
A drive unit for driving the motor comprises position sensors to detect a rotational angle position of the rotor relative to the stator, a switch circuit to switch the exciting phase of the armature coils, an accelerator operation member operated when an output of the motor should be adjusted, an acceleration sensor to output an acceleration signal having a magnitude corresponding to the opening degree of the accelerator operation member which is detected as a displacement quantity of the accelerator operation member and a controller to control the switch circuit so that the armature coils are selectively energized to commutate in accordance with the output of the position sensors which flow through the armature coils to rotate the rotor.
The controller comprises a microprocessor to perform a predetermined program, which forms duty factor arithmetical operation means to arithmetically operate a duty factor of the drive current on a value of the acceleration signal, PWM control means to control the switch circuit so as to modulate a waveform of the drive signal into a pulse width modulation waveform (PWM waveform) having the duty factor DF arithmetically operated by the duty factor arithmetical operation means and current phase angle control means to shift the switching angle of the exciting phase of the armature coils by an advance angle relative to a reference switching angle determined on the output of the position sensors.
The duty factor DF of the drive current shows the ratio of an on-time of the drive current relative to the on-off period thereof and is determined by (t on/T)xc3x97100[%] wherein xe2x80x9ct onxe2x80x9d designates a time during which the drive current flows, xe2x80x9ct offxe2x80x9d designates a time during which the value of the drive current is zero and xe2x80x9cTxe2x80x9d (=t on+t off) designates an on-off period.
In the electric vehicle, there is adjusted the output of the electric motor by displacing the accelerator operation member such as an accelerator grip or an accelerator pedal and, in order to obtain a good operation feeling of the vehicle and a smooth operation thereof, the duty factor DF of the drive current should be controlled relative to both of the displacement quantity (the opening degree xcex1 of the accelerator) and the rotational speed N of the motor whereby the change rate of the duty factor DF relative to the accelerator operation member varies in accordance with the rotational speed [rpm].
In the case where the duty factor DF is controlled relative to both of the opening degree xcex1 of the accelerator and the rotational speed N of the motor, a three-dimensional look-up table giving a relationship between the opening degree xcex1 of the accelerator, the rotational speed N of the motor and the duty factor DF of the drive current is stored in a ROM, the duty factor DF is arithmetically operated on the rotational speed N of the motor and the opening degree xcex1 of the accelerator by the microprocessor with this look-up table used and switch elements of the switch circuit are controlled so that the drive current intermittently flows in the thus obtained duty factor DF.
In the brushless DC motor, the actual switching angle (the electrical angle) for switching over the exciting phase of the armature coils is shifted just by the predetermined angle relative to a theoretical switching angle determined by the mechanical structure of the electric motor. A phase difference between the actual switching angle and the theoretical switching angle is called a current phase angle xcex3, which is generally set on an advance side.
The generation torque and the maximum rotational speed of the brushless DC motor vary on the current phase angle xcex3. As the current phase angle xcex3 is so set that the torque is larger, the maximum rotational speed gets lower and as the current phase angle xcex3 is advanced, the maximum rotational speed gets higher, but the generation torque gets lower.
Generally, in the case where the brushless DC motor is used as the drive unit of the electric vehicle, the current phase angle xcex3 by which the fully high torque can be obtained at the low speed is set as a regular current phase angle xcex3o, the current phase angle xcex3 is advanced relative to the regular current phase angle xcex3o as the rotational speed increases in the area where the rotational speed exceeds the set value, and the advance amount of the current phase angle xcex3 is held at the maximum value in the area where the rotational speed exceeds the set advance termination rotational speed at which the advance of the current phase angle is terminated.
In the case where the aforementioned control of the current phase angle is performed, a three-dimensional look-up table giving a relationship between the displacement quantity xcex1 of the accelerator operation member (the opening degree of the accelerator), the rotational speed N of the motor and the current phase angle xcex3 is stored in the ROM, the current phase angle xcex3 is arithmetically operated on the detected value of the opening degree of the accelerator and the detected value of the rotational speed by using the look-up table and the current phase angle of the electric motor is controlled so as to be equal to the arithmetically operated current phase angle.
In the case where the current phase angle xcex3 is advanced more than the regular current phase angle xcex3o in the area where the rotational speed exceeds the set value as aforementioned, when the vehicle is running uphill in the full accelerating state where the accelerator operation member is displaced to the maximum value toward the accelerating side, the advance quantity of the current phase angle is held at the maximum value and therefore the drive current of the electric motor exceeds the rated value. As this state continues for a long time, the temperature of the armature coil rises and sometimes exceeds the allowable value. Especially, as the rotational speed of the electric motor is extremely lowered on the uphill road, it takes longer for the drive current to be commuted so that the temperature of the specific phase armature coil of the electric motor abruptly rises and therefore it is possible that the armature coil is damaged by heat. As the lock state where the electric motor stops rotating on the uphill road arises, the temperature of the armature coil of the specific phase determined on the stop position of the rotor rises and the armature coil is therefore damaged by heat because the large drive current flows through the specific phase armature coil.
Thus, the prior control system for the brushless DC motor for the electric vehicle is provided with a temperature sensor to detect the temperature of the armature coils, and when it is detected by the temperature sensor that the temperature of the armature coils abnormally rises, the drive current is limited so as to restrict the output of the electric motor whereby the temperature of the armature coils is prevented from rising.
However, if the current phase angle is kept to be advanced although the drive current is limited when the temperature of the armature coils rises, a large reactive current flows through the armature coils and therefore there arises a problem that the temperature of the armature coils cannot fully be lowered. In order to solve such a problem, when the abnormal rise of the temperature of the armature coils is detected, the control in which the current phase angle is delayed and the control in which the duty factor of the drive current is lowered are performed in combination for reducing the reactive current.
Although, in the aforementioned description, the duty factor of the drive current and the current phase angle are controlled, the armature coils will be damaged by heat even though the current phase angle is not controlled when the load gets excessive in the full acceleration state so that the rotational speed of the electric motor is extremely lowered or the electric motor stops rotating. Accordingly, even though the current phase angle is not controlled, the output of the electric motor should be limited by detecting the temperature of the armature coils so that it is prevented from rising.
In the prior control system for the brushless DC motor for the electric vehicle, the temperature sensor has been provided only in one phase armature coil among the multi-phase armature coils for limiting the output of the electric motor so as to prevent the temperature from rising when the temperature detected by the temperature sensor exceeds the allowable value. However, in the case that the temperature sensor is provided only in the one phase armature coil among the multi-phase armature coils, when the temperature of the specific armature coil in which no temperature sensor is provided happens to abnormally arise, the rise of the temperature cannot be detected and therefore the armature coils cannot be positively protected.
For instance, as the brushless DC motor stops being operated due to the overload in the fully opened state of the accelerator, the drive current is never commuted and therefore more current flows through the specific phase armature coil than through the other phase armature coils, which causes the rise of the temperature of the specific phase armature coil. In this case, if no temperature sensor is provided in the specific phase armature coil, the temperature rise thereof cannot be detected and therefore the armature coils cannot be positively protected.
When the electric motor nearly stops because the rotational speed thereof gets too lower even though it does not lock and it takes too much time to commute the drive current, the large drive current flows through the specific phase armature coil for a longer time so as to get the state similar to the lock state of the electric motor, which causes the specific phase armature coil to be overheated and possibly to be damaged by heat.
Furthermore, if the overload state of the electric motor changes due to variation of road conditions when there arises the situation that the electric vehicle cannot finish running steep uphill in the fully opened state of the accelerator, there sometimes arises the so-called hunting in which the electric motor repeats a forward rotation and a reverse rotation at an extremely low speed. In case that such a phenomenon arises, the large drive current flows through the specific phase armature coil for a longer time in the same manner as the stop state of the electric motor and therefore the temperature of the armature coil possibly rises in an abrupt manner. Thus, when the hunting state arises when the electric motor rotates at the extremely low speed, the control for preventing the temperature of the armature coils from rising is desirably performed so that the hunting state is dealt with in the same manner as the lock state.
Similarly, in these cases, if the temperature of only one phase armature coil is detected, the abnormal rise of the temperature of the armature coil cannot be sometimes detected and therefore the electric motor cannot be positively protected.
It will be considered that the temperature sensor may be provided in every phase armature coil in order to detect the temperature of all the phases of the multi-phase armature coils; however, this undesirably makes the cost expensive.
If the armature coils are provided on the rotor, the temperature of the armature coils cannot be prevented from rising because it cannot be detected, since the temperature sensors cannot be provided for detecting the temperature of the armature coils.
Accordingly, it is a principal object of the invention to provide a control system for an electric motor for an electric vehicle adapted to positively protect armature coils of the electric motor by preventing a temperature of the armature coils from abnormally rising without detecting the temperature of the armature coils when the electric motor stops rotating or rotates at an extremely low speed immediately before it nearly stops rotating due to overload.
It is another object of the invention to provide a control system for an electric motor for an electric vehicle adapted to positively protect armature coils of the electric motor by preventing a temperature of the armature coils from abnormally rising even when the armature coils are provided on a rotor of the electric motor.
It is further object of the invention to provide a control system for an electric motor for an electric vehicle adapted to positively protect armature coils in an extremely low speed state where a large drive current continues to flow through a specific armature coil for a longer time by detecting a hunting state arising in the extremely low speed state as well as a state where the electric motor rotates at the extremely low speed or stops rotating in a largely opened condition of an accelerator as a lock state of the electric motor.
The invention is applied to a control system for an electric motor for an electric vehicle to control an output of the electric motor for the electric vehicle relative to an opening degree of an accelerator, which is a displacement quantity of an accelerator operation member.
The invention is made in view of an abnormal rise of the temperature of the armature coils of the electric motor for driving the electric vehicle arising when the electric motor stops due to the overload thereof or rotates at the extremely low speed in a state where the opening degree of the accelerator is enlarged to some extent. The invention is adapted to prevent the temperature of the armature coils from abnormally rising by limiting an output of the electric motor when a state where the electric motor stops rotating while the opening degree of the accelerator has a value equal to or more than a predetermined judgment opening degree or a state where the rotational speed of the electric motor has a value equal to or less than a set lock start rotational speed is detected as a lock state.
Generally, what is meant by the lock state of the electric motor is a state where the rotation of the electric motor fully stops, but in the description, the lock state includes not only the full stop state of the electric motor, but also a state where the rotational speed of the electric motor is reduced to a value extremely low value, which is treated with as the lock state.
In order to embody the invention, the control system of the invention comprises an acceleration sensor to detect an opening degree of an accelerator and output an acceleration signal having a magnitude corresponding to the detected opening degree of the accelerator, rotational speed detection means to detect the rotational speed of the electric motor, steady-state-time drive current control means to control a drive current of the electric motor in accordance with the acceleration signal, lock state judgment means to judge that the electric motor is in a lock state when a state where the rotational speed detected by the rotational speed detection means has a value equal to or less than a set start rotational speed, or zero continues for a set lock start judgment time while the opening degree of the accelerator has a value equal to or more than a set lock judgment opening degree and that the electric motor is in the unlock state when a state where the opening degree of the accelerator is less than the lock judgment opening degree or the rotational speed of the electric motor is equal to or more than a lock release rotational speed set at a value higher than the lock start rotational speed continues for a set lock release judgment time, lock-time drive current control means to perform a drive current limit control to reduce the drive current from a value determined by the steady-state-time drive current control means to a lock-time limit value to limit the maximum value of the drive current to a value equal to or less than the lock limit value when the lock state of the electric motor is judged by the lock state judgment means and to release the drive current limit control by increasing the drive current from the lock-time limit value to the value determined by the steady-state-time drive current control means when the release of the lock state is judged.
As aforementioned, as the state where the rotational speed of the electric motor is reduced to the value equal to or less than the set lock start rotational speed or the electric motor stops while the opening degree of the accelerator is equal to or more than the predetermined judgment opening degree is detected as the lock state and the drive current of the electric motor is limited to a value equal to or less than the lock-time limit value when the lock state is detected, the temperature of the armature coils can be prevented from abnormally rising by setting the lock-time limit value at an appropriate value.
In this case, it will be noted that the armature coils can be protected without detecting the temperature of the armature coils and without making the cost expensive because the lock state judgment means and the lock drive current control means can be made up by a software.
Also, according to the invention, the armature coils can be protected from overheat in the case where the armature coils of the electric motor for driving the electric vehicle are provided on a rotor because the temperature of the armature coils need not to be detected.
It should be noted that the lock start judgment time set to detect the start of the lock state should be set at a fully short value so that the temperature of the armature coils never rises within the lock start judgment time in any case.
The lock start rotational speed used for judging the start of the lock state is set at a value slightly higher than the rotational speed when the time for which the drive current continues to flow through the armature coil of each phase reaches the allowable value when the opening degree of the accelerator has the maximum value (when the accelerator is fully opened).
What is meant by the time for which the drive current continues to flow through the armature coils is a time after the drive current starts to flow through the armature coil of each phase until the drive current is commuted to the other phase and is determined by the rotational speed of the electric motor. An allowable threshold value of the time for which the drive current continues to flow through the armature coil of each phase is a conduction time required until the temperature of the armature coils reaches the allowable maximum value.
When the rotational speed of the electric motor is higher than the lock start rotational speed, the drive current is commuted with a relatively short cycle. Thus, since the time for which the drive current flows through the armature coil of each phase is fully short, the temperature of the armature coil of each phase never exceeds the allowable value even though the accelerator is fully opened.
On the other hand, when the rotational speed of the electric motor is lower than the lock start rotational speed, the drive current is commuted with a longer cycle. Thus, since the time for which the drive current flows through the armature coil of each phase gets longer, the temperature of the armature coil of each phase abnormally rises.
The lock-time limit value of the drive current is set at a value equal to or less than the maximum value of the drive current which can continue to flow through the armature coil of each phase without abnormally increasing the temperature of the armature coil of each phase and enough to be able to generate a certain degree of torque from the electric motor.
As the torque continues to be generated from the electric motor by passing a certain amount of the drive current through the armature coils even though the electric motor is in the lock state as aforementioned, the vehicle can be prevented from running in a reverse direction by losing the torque even though it stops on a slope due to the lock state of the electric motor, for example.
The lock-time drive current control means may be preferably formed so as to perform the drive current limit control and the release thereof as described hereinbelow. More particularly, the lock-time drive current control means may be preferably formed so that it gets the lock mode when the electric motor is in the lock state to perform the drive current limit control to limit the maximum value of the drive current to a value equal to or less than the lock-time limit value by gradually decreasing the value of the drive current from a value determined by the steady-state-time drive current control means to the lock-time limit value over a set lock start control time and gets the lock release mode when it is judged that the electric motor is released from the lock state to release the drive current limit control by gradually increasing the value of the drive current from the lock-time limit value to the value determined by the steady-state-time drive current control means over a set lock release control time.
As the lock-time drive current control means made up in the aforementioned manner gradually decreases the drive current of the electric motor to the lock-time limit value when it gets the lock state and gradually increases the drive current when it is released from the lock state, the control to limit the drive current for the protection of the armature coils and the release thereof can be performed without any abrupt variation in the torque and therefore the protection operation can be accomplished without surprising the driver.
A brushless DC motor which is equipped with a rotor having a field system and a stator having n phase (n is two or more integers) armature coils is used well for the electric motor for driving the electric vehicle. With such an electric motor used, there is provided steady-state-time drive current control means including n position sensors to directly or indirectly detect the magnetic poles of the rotor relative to each of the n phase armature coils of the stator to generate an output having level varying whenever the polarity of the detected magnetic pole changes, a switch circuit provided between the DC power source and the armature coils to switch the exciting phase of the armature coils where the drive current flows from the DC power source, duty factor arithmetical operation means to arithmetically operate a duty factor of the drive current for the acceleration signal and switch control means to control the switch circuit so that the drive current of PWM waveform continuing in the duty factor obtained by the duty factor arithmetical operation means flows through the armature coil of phase determined in accordance with the output of the position sensors to rotate the rotor as well as the acceleration sensor to detect as the opening degree of the accelerator the displacement quantity of the accelerator operation member operated when the output of the electric motor is adjusted and the rotational speed detection means to detect the rotational speed of the electric motor.
In the case where such a brushless DC motor is used as the electric motor for driving the electric vehicle, the construction described hereinbelow is used so that the armature coils can be protected by detecting as the lock state a state where the hunting arises due to the extremely low speed of the electric motor as well as the state where the rotational speed of the electric motor is lowered.
More particularly, in the case where the hunting state as well is detected as the lock state, there are provided rotation state judgment means to judge whether the electric motor rotates in a forward direction by using the fact that variation in combination of the output state of the n position sensors (an output pattern of the n position sensors) is different on the forward rotation of the electric motor and the reverse rotation thereof, lock state judgment means to perform every predetermined time, judgment steps in which the lock mode of the electric motor is judged when the rotation state judgment means judges that electric motor rotates in the forward direction while the opening degree of the accelerator is equal to or more than the lock judgment opening degree and the state where the rotational speed is equal to or less than the set lock start judgment rotational speed, or zero continuers during a set lock start judgment time or when the state where it is judged that the electric motor does not rotate in the forward direction while the opening degree of the accelerator is equal to or more than the lock judgment opening degree continues during the set lock start judgment time and the release of the lock mode is judged when the opening degree of the accelerator is less than the lock judgment opening degree or when the state where the rotational speed of the electric motor is equal to or more than the lock release rotational speed set at a value higher than the lock start rotational speed continues during a set lock release judgment time and lock-time drive current control means to perform a drive current limit control to limit the maximum value of the drive current to a value equal to or less than the lock-time limit value when the lock mode of the electric motor is judged by the lock state judgment means and to release the drive current limit control when it is judged that the lock state is released.
With the aforementioned lock state judgment means provided, when the rotational direction of the electric motor is going to be reversed due to the hunting at the extremely low rotational speed of the electric motor, it is judged as the lock state so as to limit the drive current. Thus, it will be noted that the armature coils can be positively protected when the state where the large drive current flows through the armature coil of the specific phase due to the hunting continues.